Journal of Mechanical Engineering and Biomechanics

Transcription

1 Volume 1 Issue 1, Page Journal of Mechanical Engineering and Biomechanics Analysis of internal Flow and cavitation in diesel injector nozzle Vimal Kumar Pathak *, Shavetabhra Shukla ** * Department of Mechanical Engineering, MNIT Jaipur;Rajasthan;India, 2013rme9053@mnit.ac.in Abstract Research Article This study analyses the internal nozzle flow, the cavitation and its effect in the performance of diesel injector nozzle. Modeling has been carried out by the modification in the internal geometry of the injector nozzle with the help of the Computational Fluid Dynamics (CFD) interface. The effect on internal flow, turbulence, pressure and velocity has been assessed by modifying included angle of the injector. It was found that as the liquid fuel injection pressure has increased above 180 MPa, the effects of the cavitation in the nozzle and in the spray process shows significant increase. Article History Received 24/05/2016 Revised 03/07/2016 Accepted 05/07/ Published by rational publication. All rights reserved Keywords: Diesel injector, cavitation, CFD, injector nozzle, mini-sac and internal flow 1. Introduction In present scenario the diesel engines are widely used in automobile industry, power generation, mining industries and various other industrial applications. The performance and emission characteristics of diesel engines are governed by fuel atomization and spray processes which are controlled by the flow dynamics inside the injector nozzle. A good understanding of the flow inside the nozzle is essential for predicting spray development. This is particularly true when cavitation occurs inside the nozzle. Most of the improvements are done in fuel injection systems to improve their efficiency and reduce the emissions of diesel engines. One of the most commonly used strategies in recent years has been the increase of injection pressure, which has meant that cavitation and turbulence play a fundamental role on the internal flow and spray development [1],[2]. When a liquid is subjected to a pressure below its saturation value at a given temperature, it begins to evaporate. This phenomenon is called cavitation or cold boiling [3]. Cavitation can occur when a fluid with high velocity passes through a contraction; due to the abrupt change in flow direction, the flow tends to separate from the wall at the inlet section. As a consequence, a recirculation phenomenon appears accompanied by a pressure fall due to the acceleration of the fluid. If conditions in the nozzle are such that the static pressure decreases under the saturation pressure of the working fluid, a local change of state from liquid to vapour takes place; this phenomenon is called hydrodynamic cavitation [4]. At the time of injecting fuel into the combustion chamber in modern diesel engine cavitation often occurs inside the fuel injector nozzle. This also causes vapour bubbles to flow and increases the maximum velocity in the liquid core. The most acceptable reasons of that phenomenon are, firstly, if there is vapour along the wall, the liquid will have a slip condition boundary, thus allowing the velocity of the liquid to increase 5. Moreover due to the formation of vapour bubbles the liquid cannot fill the entire channel (geometrical diameter), and so the diameter (effective) is reduced with regard to the geometrical one[5],[6]. Furthermore, cavitation increases the spray cone angle and so it is *Corresponding Author : Vimal Kumar Pathak Address: 2013rme9053@mnit.ac.in

2 expected to improve the air, fuel mixing process [5]-[7]. In this paper a study on the internal flow characteristics of fuel injector of diesel engine has been done with the help of CFD analysis. 2 Methodological approach In order to analyze the internal nozzle flow in CFD FLUENT 6.1 software is used. CFD software has two tools FLUENT and Gambit2.1.6.Mathematical modeling of continuum problem leads to a set of differential, integral or integro- differential equations. Exact analytical solution of such equation is limited to problem in simple geometries. For most of the problem of practical interest an approximate numerical solution is sought. CFD deals with approximate numerical solution of governing equations based on the fundamental conservation laws of physics, namely mass, momentum and energy conservation. The CFD solution involves: Conversion of governing equations for a continuum medium into a set of discrete algebraic equations using a process called discretization. Solution of the discrete equations can using a high speed digital computer to obtain the numerical solution to desired level of accuracy. In numerical simulations of internal nozzle flow, three approaches are mainly considered: Interface tracking methods. These models only solve the physical equations in the liquid phase and assume that the pressure in cavities is equal to that of the vapour pressure of the liquid. In this way, the liquid vapour interface is explicitly tracked in conjunction with a wake model to handle the cavity closure region and predict the shape of the cavity. Interface tracking models normally use grid regeneration (adaptive grids) to conform to the cavity shape and has been demonstrated to work reasonably well for steady state sheet cavitation [8], but require extensive reworking for cases of bubble growth, bubble collapse and bubble detachment. Two-fluid nozzle flow models, which treat the liquid and vapour phases separately, i.e. two sets of governing equations (one for each phase) are solved and interaction between the phases are modelled by an additional source term. This model can be grouped in two mainly categories: Eulerian Eulerian models [9],[10] and Eulerian Lagrangian models [11],[12] The Eulerian Eulerian models are based on the transport of volume fraction, and a source term representing phase transition that is governed by the difference between local pressures and vapour pressure. Cavitation phenomenon is assumed to take place due to the presence of bubble nuclei within the liquid phase, which can grow or collapse and are taken into account by the Rayleigh s simplified bubble dynamics equation. On the other hand, the Eulerian Lagrangian models consider liquid as the carrier phase in a Eulerian frame of reference and vapour bubbles as the dispersed phase using a Lagrangian frame of reference, using bubble parcels to simulate the entire population of actual vapour bubbles. In order to start the cavitation phenomenon, nuclei are artificially created defining the size of each nucleus from a probability density function. For these models the bubble dynamics is calculated using the complete Rayleigh Plesset equation. Continuum nozzle flow models or homogeneous equilibrium models [13]-[15] henceforth mentioned as HEM, on which it is assumed that liquid and vapour phases are always perfectly mixed in the sub-grid cell and also the temperature is constant. These homogeneous equilibrium models are most widely used and have various forms depending on how the equation of state and pressure equation are formulated. In these models, an equation of state relating pressure and density (barotropic equation) allows the calculation of the growth of cavitation to be done [16]. 3 Result and discussion The internal geometry of a Bosch CRI1 common-rail injector was modelled, as shown in Fig. 1. Nozzle type is mini-sac and has 5 equally spaced holes, with outlet diameter of 130 µm and hole length of 700 µm. The hole shape studied is a cylindrical orifice shape (k-factor = 0). Exploiting the symmetry of the injector, only one tenth of the fluid domain, i.e. 36 sector, was modelled in 2D [17]. The modification is done in the angle between the nozzle axis and the 39

3 cylindrical fuel delivery hole axis. Initially the angle between nozzle axis and cylindrical hole axis is 75ᵒ and the angle is modified by +5ᵒ and -5ᵒ angle is changed to 80ᵒ and 70ᵒ as shown in Fig. 2. Fig.1 Section of nozzle (a) (b) (c) Fig. 2 Mesh view (a) 70 o. (b) 75 o and (c) 80 o The properties of diesel at ambient temperature are shown in Table 1. The properties include density, dynamic viscosity and surface tension as required for the analysis part in Fluent software in both liquid and vapour phase. The boundary conditions are depicted in Table Table 1. The properties of diesel fuel at ambient temperature [17] Liquid Phase Vapour Phase Density (kg/m 3 ) 825 Density (kg/m 3 ) - 5 Dynamic viscosity (Pas) Dynamic viscosity (Pas) Surface Tension (N/m) Saturation pressure (Pa) 1000 The initial pressure of fluid at the inlet is P in = 80 MPa and the back flow pressure is 9 MPa. The pressure of fluid decreases as the area of cross-section reduces, at the entry of the hole orifice there is further more reduction in pressure due to the cavitation as seen in Fig. 3 and 4. Zoom view of the nozzle injector pressure reduction is shown in Fig. 5. Cavitation occurs in the holes of injector nozzle. When the pressure of the liquid falls below its saturation pressure for

4 a given temperature, controlled cavitation is required for the injector nozzle as it helps in the better mixing of the fuel as it induces turbulence and small droplets are formed which easily evaporates. On the other hand cavitation decreases the discharge coefficient and can do severe damage to the nozzle. Table 2. Boundary Conditions [16] Condition at Inlet Condition at outlet Injection pressure - 80 MPa Back pressure - 9 MPa Temperature K Vessel back Temperature 577 K (a) (b) Fig. 3 (a) Contours of pressure for 75 o,(b) 70 angle between nozzle axis and cylindrical fuel delivery hole 41

5 Fig. 4 Contours of pressure for 80 o angle between nozzle axis and cylindrical fuel delivery hole Fig. 5 Cavitation comparison in hole length 70 o and 80 o angle between nozzle axis and cylindrical fuel delivery hole In the below Fig. 6 and 7, the static pressure contours of cylindrical holes are compared the pressure reduction at the hole orifice in 80 o nozzle is more than the 70 o nozzle. The low pressure zone indicated by dark blue is more in 80 degree nozzle means the is a presence of fuel vapour inside the flow along the walls indicates the 80 o aligned cylindrical hole of the nozzle with nozzle axis is more cavitating then 70 o aligned cylindrical hole of the nozzle with nozzle axis. Fig. 8 shows the contours of velocity in hole length due to cavitation for 70 and 80 angle. The velocity is at the inlet is low as there is initial pressure is very high as fluid moves the velocity of the flow increase due the reduction in the cross- sectional area of the profile and at the inlet of the orifice of cylindrical hole the velocity of the fluid flow increases due the presence of cavitation. The velocity increases when the fluid is cavitating for two reasons Frictional losses will be reduced due to the vapour along the wall allowing the velocity of liquid phase to increase as there is large difference in viscosities of vapour and liquid. When the fluid is cavitating the liquid phase cannot fill the entire channel and so the effective diameter is reduced compared to the geometrical diameter thus increasing in velocity of flow. 42

6 (a) (b) Fig. 6 Contours of velocity for (a) 70 o and (b) 75 angle between nozzle axis and cylindrical fuel deliveryhole Fig. 7 Contours of velocity for 80 o angle between nozzle axis and cylindrical fuel delivery hole 43

7 44 Fig. 8 Contours of velocity in hole length due to the cavitation, for 70 o and 80 o angle In the above Fig. 8 the velocity contours of cylindrical holes are compared the increase in velocity at the hole orifice in 80 o nozzle is more than the 70 o nozzle. The higher velocity is shown by the red and the orange colored zone which is more darker in the 80 o aligned cylindrical hole of the nozzle with nozzle axis is more cavitating then 70 o aligned cylindrical hole of the nozzle with nozzle axis. 4 Conclusions The cavitation phenomenon can lead to the premature failure or reduction in useful operating life although cavitation is a desirable phenomenon in an injector nozzle within certain limit.from the comparison of the above results it shows that the nozzles with delivery hole inclined at 70 o with the nozzle axis are less cavitating than the nozzle which has their delivery hole inclined at 80 o with the nozzle axis. References [1] Pathak VK, Gupta S. Study of nozzle injector performance using CFD. IJMECH Aug;4(3): [2] Suh HK, Lee CS. Effect of cavitation in nozzle orifice on the diesel fuel atomization characteristics. International journal of heat and fluid flow Aug 31;29(4): [3] Patouna S. A CFD study of cavitation in real size diesel injectors (Doctoral dissertation). [4] Salvador FJ, Romero JV, Roselló MD, Martínez-López J. Validation of a code for modeling cavitation phenomena in Diesel injector nozzles. Mathematical and Computer Modelling Oct 31;52(7): [5] Chaves H, Obermeier F. Correlation between light absorption signals of cavitating nozzle flow within and outside of the hole of a transparent diesel injection nozzle. Proc. ILASS-EUROPE. Manchester, UK Jul:6-8. [6] Schmidt DP, Corradini ML. The internal flow of diesel fuel injector nozzles: a review. International Journal of Engine Research Feb 1;2(1):1-22. [7] Payri F, Bermudez V, Payri R, Salvador FJ. The influence of cavitation on the internal flow and the spray characteristics in diesel injection nozzles. Fuel Mar 31;83(4): [8] Payri R, Salvador FJ, Gimeno J, De la Morena J. Study of cavitation phenomena based on a technique for visualizing bubbles in a liquid pressurized chamber. International Journal of Heat and Fluid Flow Aug 31;30(4): [9] Liu L, Li J, Feng Z. A numerical method for simulation of attached cavitation flows. International journal for numerical methods in fluids Oct 30;52(6): [10] Singhal AK, Athavale MM, Li H, Jiang Y. Mathematical basis and validation of the full cavitation model. Journal of fluids engineering Sep 1;124(3): [11] Yuan W, Schnerr GH. Numerical simulation of two-phase flow in injection nozzles: Interaction of cavitation and external jet formation. Journal of Fluids Engineering Nov 1;125(6):963-9.

8 [12] Giannadakis E, Gavaises M, Roth H, Arcoumanis C. Cavitation modelling in single-hole diesel injector based on Eulerian-Lagrangian approach. InProc. THIESEL International Conference on Thermo-and Fluid Dynamic Processes in Diesel Engines. Valencia, Spain 2004 Sep 11. [13] Liu TG, Khoo BC, Xie WF. Isentropic one-fluid modelling of unsteady cavitating flow. Journal of Computational Physics Nov 20;201(1): [14] Habchi C, Dumont N, Simonin O. Multidimensional simulation of cavitating flows in diesel injectors by a homogeneous mixture modeling approach. Atomization and sprays. 2008;18(2). [15] Kärrholm FP. Numerical modelling of diesel spray injection, turbulence interaction and combustion. Chalmers University of Technology; [16] Payri F, Payri R, Salvador FJ, Martínez-López J. A contribution to the understanding of cavitation effects in Diesel injector nozzles through a combined experimental and computational investigation. Computers & Fluids Apr 15;58: [17] Battistoni M, Grimaldi CN. Numerical analysis of injector flow and spray characteristics from diesel injectors using fossil and biodiesel fuels. Applied Energy Sep 30;97: